Vehicles have an engine and various accessories such as an air conditioning (A/C) compressor, power steering pump, or alternator intermittently coupled to the engine. The A/C compressor is coupled to the engine via the A/C compressor clutch. When the A/C compressor is engaged, it consumes a certain amount of engine combustion torque. When this happens, a vehicle driver can feel the impact of A/C compressor engaging, and thus drive feel is degraded. Better engine control and improved drive feel are achieved by estimating A/C compressor load torque and adjusting engine torque accordingly. In other words, by increasing engine torque in response to increased accessory load torque, degradation in vehicle performance is avoided, and driver comfort during A/C compressor engagement is improved.
One control method of this kind is described in U.S. Pat. No. 5,826,200. engine output torque is calculated by using engine characteristics, and torque converter torque is calculated using torque converter characteristic. In the low-speed, light-load vehicle operation region the impact of the load torque of accessory devices is relatively large compared with the output torque of the engine. Therefore, when the speed ratio is relatively small, accessory load torque can be calculated as the difference between the output torque of the engine and the torque converter torque.
The inventors herein have recognized a disadvantage with this approach. In particular, when the torque converter torque is interpolated from the characteristic map of the torque converter ratio as related to the speed ratio, variations in the torque converter fluid characteristics can cause reduced accuracy. In other words, estimating torque using torque converter characteristics is susceptible to variations in hydraulic fluid properties. For example, when the fluid is cold, it is more viscous, and torque prediction will not be as accurate. Thus, accessory torque estimates are affected by the variation in hydraulic fluid properties.
Another disadvantage is that this method does not account for the torque due to losses from friction work, pumping work, accessory load other than A/C, etc. The above approach does not distinguish between the above "loss" torques and A/C load torque. Variations in this "loss" torque can cause further inaccuracies in estimated accessory load torque. Additionally, the above method cannot be used when the torque converter is locked, which is a large proportion of the vehicle drive cycle.